1970 Heater Box

[Tom L.]

I think I know the answer to this, but I'll ask anyway rather than making a mistake. The engine compartment was blacked out without the heater box in place, right?

I've seen many restored cars with black paint on the box, as mine has on the top and front. However, the book says no paint.

Also, I know the wire and hose brackets were in place during blackout, but what about the firewall grommets?

Last question, can anyone recommend a heater box/firewall sealant material that looks like the factory stuff? Thanks.

[John H.]

Re: 1970 Heater Box

I think I know the answer to this, but I'll ask anyway rather than making a mistake. The engine compartment was blacked out without the heater box in place, right?

I've seen many restored cars with black paint on the box, as mine has on the top and front. However, the book says no paint.

Also, I know the wire and hose brackets were in place during blackout, but what about the firewall grommets?

Tom -

The heater box was installed on the Hard Trim Line, LONG after the Paint Shop; it shouldn't have any blackout paint on it. Same with the firewall grommets - they were installed with the hoses/wires that went through them, also on the Hard Trim Line.

[Tom L.]

Re: 1970 Heater Box

Thanks John. Any suggestion on the sealant?

[Don L.]

Re: 1970 Heater Box

Didn't want this interesting issue to get too far down the list without closure. Thanks for the blackout guidance.

Any ideas as to best sealant? There appear to be at least 2 of us looking to learn. Tom - thanks for the post!

[Harry S.]

Re: 1970 Heater Box

My suggestion is to avoid the sealant. Since the car is probably not driven in harsh conditions and probably very few miles every year I don't believe you need the sealant. The foam gaskets on the inside and outside should suffice.

Also, if you ever want to pull the heater box in the future, not having any sealant will make the job much easier.

When I rebuilt my 63 inner and outer box, I found no evidence of sealant on either side of the firewall.

[Chuck S.]

Re: 1970 Heater Box

Didn't want this interesting issue to get too far down the list without closure. Thanks for the blackout guidance.

Any ideas as to best sealant? There appear to be at least 2 of us looking to learn. Tom - thanks for the post!

I also have observed black paint around the flange area of the heater box where it fastens to the firewall, and mentally wrestled with the variance from the JM. But, as John said, it would have been impossible for this "blackout" to have been applied during initial assembly at the factory.

In my opinion, there are two possible explanations for black overspray on heater box flange areas of fairly original cars: (1) It happened at the factory after they had to fix water leaks or fix appearance defects before shipping (less likely but the frequency and simularity of appearance makes me wonder), or (2) It was applied as engine compartment beautification by proud, but unskilled, former owners (more likely over the decades).

If I recall correctly, the heat box or AC evaporator assembly was sealed to the firewall by a combination of two methods: A pliable "dum-dum" type sealant was used between the HVAC box flange and the firewall, and a thick liquid asphalt-based sealant was dribbled/sprayed over the top of the flange/firewall joint. You can confirm my recollection by close study of the applicable AIM pages...the product desciptions and quantities to be used will be detailed there. Three M's "Strip-Calk" will be a close match to the original dum-dum sealant used between the flange and firewall (Again, confirm my memory using the AIM).

Close observation of original cars reveals a crusty, bubbly, black asphalty residue on the top of the heater box/firewall joint, as well as all firewall penetrations (grommets, electrical junction), and in the plenum area, including wiper transmission mounting flanges. In the southwest, where evaporative cooling is still widely used, home stores sell a product for waterproofing evaporative cooler pans...it's a pretty close appearance and functional match for what GM used in the 70s. This product is intended to be brushed on cold (but could be "dribbled"), whereas, I suspect the bubbly nature of the residue probably indicates GM's sealant was applied hot. A thorough judging would expect this sealing to be on the firewall, and proper appearance will be the primary purpose of this sealant.

[Andrew P.]

Re: 1970 Heater Box

I know that "WURTH" has a product that looks like mac-tac that's in a roll. I have used it in the past for this job and it works great.

I'll have a look to see if I still have any left, as I could give you a part number or product name.


Andrew (48361)

[Don L.]

Re: 1970 Heater Box

Thanks, guys. Very, very useful information and much appreciated.